An object of the invention is to provide a new type of form leading to self-heating of the concrete and whose fabrication is much simpler, much more rapid and more economical and whose operations are easier than those of conventional forms which necessarily include a rigid and heavy framework to resist, without deformation, the substantial pressure of the concrete at the time of casting.
To succeed in the results, the invention proposes a form comprising:
A casting wall of sheet steel; PA1 A rear wall also of sheet steel, the said casting wall facing the rear wall and being spaced therefrom; and PA1 A layer of expanded rigid polyurethane foam of high density molded in situ between the said walls and which achieves the connection between the said walls, the mechanical properties being such that the assembly of the form behaves under the force of the concrete as a beam and as a consequence: PA1 The layer of polyurethane must have a sufficient rigidity in order to prevent relative displacement of the two walls and to resist shear-tensile forces, and for this purpose it is expanded at least to 200 kg/m.sup.3 ; PA1 The steel casting wall must have a thickness of at least 2.5 mm to resist the tensile forces; PA1 The steel rear wall must have a thickness of at least 2.5 mm to resist the compression forces; and PA1 The adherence of the polyurethane layer with the said walls must have mechanical properties at least equal to that of said layer, i.e. it must be able to resist shear-tension forces.
An adherence presenting such properties can be advantageously obtained in the above noted manner by reason of the molding, in situ, of the polyurethane between the two plates due to the adherence properties of the polyurethane after polymerization.
Of course, it is possible to increase the adherence by coating the interior surfaces of the two walls before the molding with a primary adhesive.
The panels according to the invention are generally mounted at regular intervals on vertical posts of a conventional framework. Although these posts are formed as profiled steel members and despite the utilization of props or other supports, the subject, at the time of casting of the concrete, to flexure forces which according to the height of the posts can be translated at their upper portion to a deflection of the order of 0.5 cm.
Furthermore, despite its rigidity and its behavior as a conventional beam under the pressure of the concrete, the panel itself is subjected to a flexure in the space between two consecutive posts, to produce a bending deflection which can reach 0.15 cm. As a consequence, the molded face of the concrete article will not be perfectly planar.
A further object of the invention is to eliminate these disadvantages. For this purpose, the invention contemplates a construction in which the casting wall of the panel is not planar but is initially shaped such that when the panel is subjected to the pressure of the concrete, the deformations of the panel and that of the framework will be compensated to obtain a cast wall which is exactly planar.
According to the invention, the rear wall can be planar and come into contact over its entire height with the posts of the framework. The rear wall can also be reinforced by a succession of horizontal parallel folds at spaced intervals which decrease from top to bottom to take into account the pressure gradient of the concrete. This embodiment permits utilization of expanded polyurethanes of lower densities, for example, of the order of 100 kg/m.sup.3.
It is to be noted that in all cases the panel according to the invention presents the advantage of possessing a very low thermal conductivity, notably because there is no thermal conduction path between the walls. This thermal insulating property permits obtaining a self-heating of the concrete by the substantial magnitude of heat generated during the curing thereof, and this accelerates the curing and permits a very rapid setting of the concrete.
Several embodiments of the invention will be described hereafter by way of non-limitative example with reference to the attached drawings.